Heat Of Liquefaction Research Articles

Moisture permeability, diffusion and sorption in organic film‐forming materials

AbstractExperimental results which have been obtained in the course of comprehensive investigations of the water‐vapour permeability, diffusion and sorption of organic film‐forming materials are summarized. These data are used for elucidating the mechanisms and inter‐relationships of the processes. The analysis is based on thermodynamic principles and the multimolecular theory of adsorption. It is shown that the moisture sorption/relative humidity curves agree with the B.E.T. (Brunauer, Emmett and Teller) adsorption equation up to about 90% R.H. The heat of sorption is found to be the same as the heat of liquefaction of free water. It is concluded that the moisture sorption is a result of hydrogen‐bond formation with hydrophilic radicals in the molecules of the solid plus van der Waals' forces. The number of adsorption sites per g. of solid is calculated. The entropies of sorption for the various materials are estimated to show the degree of order of the sorbed moisture as compared with ice.Diffusion constants are derived from sorption‐ and desorption‐rate curves; an anomaly in some of the sorption‐rate curves is discussed. The activation energies of diffusion indicate that, with one exception, diffusion is a process of repeated vaporization and sorption; the energy required for ‘hole’ formation is relatively low except with polyvinyl chloride (PVC). Fick's law of diffusion is not applicable at medium and high R.H.s; it is probable that the rate of permeation depends directly on the vapour‐pressure difference and not on the concentration gradient, but it appears that quantitative calculation of sorption and desorption rates by the classical diffusion equation is not very sensitive to appreciable deviations from Fick's law. The derivations of the B.E.T. equations for multimolecular adsorption and for the free energy of sorption are given as Appendices.

Interaction between Polymers and Fillers

Abstract In the literature the problem of whether or not filler particles adhere to a polymer is still under discussion. This point is already of interest in the mixing process because here the question resolves itself into whether the particles clog together or each becomes separately surrounded by polymer only. The problem is also of paramount interest with regard to the properties of the final mixture because upon the interaction depends whether or not a filler has a reinforcing action. This is a key problem, especially in the rubber industry. Essentially, the problem of dispersing a filler in a polymer is of the same type as that of dissolving a polymer in a solvent, and for the thermodynamic considerations we know that dispersing occurs if (leaving the entropy factor out of discussion) ΔU(U=internal energy) is negative, that is, heat is liberated. Here the historical measurements of Hock and his coworkers show that, on mixing rubber with carbon black, 11 gcal. per gram of black (which is of the order of 1 gcal. per mole) extrapolated to zero concentration, are developed. This heat is found to decrease with increasing proportion of filler, showing that all particles are no longer in contact with the rubber because of clogging. It has long been a problem to explain the outstanding reinforcing properties of carbon black with the aid of these low values found by Hock. Smith and Schaeffer showed that the initial heat of adsorption between carbon black and C4 hydrocarbons is of the order of magnitude of 15 kcal. per mole, decreasing sharply until about 40 per cent of a monolayer is formed. This indicates that 40 per cent of the carbon black surface is covered with sites of high adsorptive capacity. Between 40 and 100 per cent of this monolayer formation the surface appears to be quite uniform with regard to adsorptive capacity; at the monolayer the values again decrease and approach the heat of liquefaction, EL, of the adsorbate. These results are shown in Figure 1. From the data obtained, it can be derived that the heat of adsorption per CH2 group to carbon black is about 4 kcal./mole.

Transport and Equilibrium Phenomena in Gas-Elastomer Systems. II. Equilibrium Phenomena

The solubility of nitrogen, ethylene, and the n-paraffins from methane to pentane has been measured in a series of natural rubber vulcanizates in relation to chain length of paraffin, temperature, and degree of cross linking of vulcanizates which contained between 1.7 to 21.9% combined sulfur. For smaller, less soluble paraffins the degree of vulcanization had little influence upon the solubility but for higher molecular paraffins such as pentane this influence became significant. A qualitative interpretation of the effects observed was given. The solubility data are in all cases represented by a statistical theory of one of the authors for a mean frequency of vibration of the solute in the medium corresponding to the infrared (0.5 to 1.0 × 1012 sec.−1). The solubility constants, σ, and critical temperature, Tc, are also empirically related by the equation: Heats and standard free energies of solution for the earlier homologous paraffins show steady trends as the chain length increases. The heats are exothermal and are adequately interpreted as the sum of the heats of liquefaction together with a small heat-of-mixing term for liquid and rubber which was usually close to that given by Hildebrand's cohesive energy density equation: As anticipated from statistical theory, entropies were usually several entropy units more negative than those for solutions of gases in monomeric solvents.

Transport and equilibrium phenomena in gas–elastomer systems. II. Equilibrium phenomena

AbstractThe solubility of nitrogen, ethylene, and the n‐paraffins from methane to pentane has been measured in a series of natural rubber vulcanizates in relation to chain length of paraffin, temperature, and degree of cross linking of vulcanizates which contained between 1.7 to 21.9% combined sulfur.For smaller, less soluble paraffins the degree of vulcanization had little influence upon the solubility but for higher molecular paraffins such as pentane this influence became significant. A qualitative interpretation of the effects observed was given. The solubility data are in all cases represented by a statistical theory of one of the authors for a mean frequency of vibration of the solute in the medium corresponding to the infrared (0.5 to 1.0 × 1012 sec.−1). The solubility constants, σ, and critical temperature, Tc, are also empirically related by the equation: Heats and standard free energies of solution for the earlier homologous paraffins show steady trends as the chain length increases. The heats are exothermal and are adequately interpreted as the sum of the heats of liquefaction together with a small heat‐of‐mixing term for liquid and rubber which was usually close to that given by Hildebrand's cohesive energy density equation: As anticipated from statistical theory, entropies were usually several entropy units more negative than those for solutions of gases in monomeric solvents.

Visible Adsorbed Films and the Spreading of Liquid Drops at Interfaces

THE adsorption isothermals of benzene and methyl alcohol vapours at the surface of mica are of sigmoid form, concave to the pressure axis until the first monolayer is nearing completion, and thereafter becoming more and more steeply inclined to it as saturation is approached. Though the adsorption energy decreases on completion of the first monolayer to a value very near the normal heat of liquefaction, the polymolecular films formed at saturation have properties quite different from the bulk liquids. This is strikingly demonstrated in a series of experiments we have recently carried out, a few of which are briefly recorded in this note.